Ultrasonic energy is commonly employed to securely interconnect portions of regularly or irregularly shaped workpieces. In ultrasonic welding processes, localized heating and softening of the workpiece being welding is induced by ultrasonic vibrations communicated thereto. Although ultrasonic welding is a process particularly suited for joining thermoplastic materials, since it is relatively fast, generates no fumes or waste material, and results in an extremely durable bond, the principles of the present invention are applicable to interconnect two workpieces of other materials as well.
In a typical ultrasonic welding system, acoustic energy is produced from electromagnetic energy by means of a transducer, also referred to as a converter. The transducer includes piezoelectric material as its action element which is adapted to produce mechanical vibrations when an electric field is shunted thereacross. Operatively connected to the transducer is a source of alternating current which transmits the electrical field across the piezoelectric material. Typically, radio frequency (hereinafter referred to as r.f.) energy of 20 kilohertz is transmitted to the transducer by a r.f. power generator, and accordingly, the piezoelectric material viabrates at the same 20 kilohertz frequency.
The welding operation is carried out by means of a welding tool generally referred to as a welding horn. The welding horn must make solid mechanical contact with both the transducer which supplies the acoustic energy thereto and the workpiece to which it transmits ultrasonic viabrations. The welding horn is usually shaped and massed into a particular configuration designed to optimize the transfer of ultrasonic vibrations to the workpiece; accordingly, its length, width, shape and mass determine the necessary frequency and waveform of the acoustic energy, as well as the physical properties (e.g. dimensions, material, thickness etc.) of the workpiece. It is usually necessary to employ differently configured welding horns to accomplish various welds on a single workpiece. Because of the degree of mechanical vibration and thermal stress to which the welding horns are exposed, they are formed of durable materials such as titanium and the like.
It is advantageous in many instances to employ a "booster", also referred to as a velocity transformer, in conjunction with an associated transducer and welding horn pair. The booster is fixedly attached to the transducer so as to make solid mechanical contact both with the transducer and the welding horn assembly. It is a function of the booster to modify the amplitude of the acoustic energy generated by the transducer, and thereby individually tailor the ultrasonic vibrations ultimately transmitted by the welding horns to the workpiece. The booster is a generally cylindrical, solid member of a particular shape to provide a specific mass; the shape and mass being chosen so that the booster will convert an acoustic wave pattern inputted thereinto by the transducer, into an output ultrasonic vibration transmitted to the welding horn. While the booster does not change the frequency of the vibration, it is adapted to adjust the amplitude thereof, i.e. the distance the vibrating welding horn will travel in each oscillatory cycle.
Further control of the ultrasonic vibrations delivered to the workpiece is accomplished by the use of a tuning coil operatively connected to the r.f. power generator. The tuning coil matches the impedence of the r.f. power generator to produce the optimum ultrasonic signal, taking into consideration the impedence characteristics of the transducer, booster, welding horn, and workpiece. Through the selection of the proper booster/welding horn combination, combined with preselected tuning of the tuning coil, the amount of ultrasonic energy delivered to the workpiece may be adjusted to the predetermined level. If the r.f. signal from the r.f. power generator were not tuned, the ultrasonic vibration transmitted to the workpiece could either produce a weak weld or damage the workpiece.
The foregoing description of ultrasonic welding has been made with reference to a single welding horn welding system. Obviously, in commercial assembly line processes it is necessary to substantially simultaneously perform a plurality of welding operations upon a workpiece. Accordingly, most commerically available ultrasonic welding systems employ a plurality of welding horns, each energized by its own r.f. power generators via an associated transducer. The utilization of such multihorn apparatus, while resulting in a significant savings of time, are quite expensive since they are essentially an aggregation of single welding horn welding systems. Obviously, if some of the components of the system could be eliminated, a significant savings of space and money would result.
More particularly, the r.f. power generator used to energize the transducers are quite expensive and somewhat bulky. Accordingly, if a single r.f. power supply were specially adapted to energize a plurality of welding horns in a multihorn welding system, the aforementioned savings of space and money would result.
One such multihorn, single r.f. power generator system, albeit not an assembly line system, is described in U.S. Pat. No. 3,830,524 of Abildgaard, entitled "Book Bound By Ultrasonic Means". In the Abildgaard system, a plurality of identically configured welding horns are energized by a single transducer via a plurality of waveguides secured thereto. Abildgaard utilizes a single r.f. power generator and a single transducer to energize the plurality of welding horns. While the system described by Abilgaard does provide a multihorn welding system having a reduced number of components, the patent fails to teach the manufacturing industry how to reduce the number of components in a welding apparatus which requires a plurality of multiconfigured and multimassed welding horns. Because the welding horns of Abildgaard are identically shaped and weighted, no teaching is found therein for individually tuning the ultrasonic vibrations provided by each welding horn to the workpiece. A final restriction on the adaptability of the Abildgaard welding system to larger, industrial welding systems is one of power; since a plurality of welding horns are being simultaneously energized by a single transducer, there are limitations on the power which may be simultaneously delivered to each. For the foregoing reasons, the Abildgaard system is limited to those applications in which the r.f. signal is supplied to a single transducer which energizes a plurality of indentically shaped and massed welding horns to transmit identical doses of relatively low amounts of ultrasonic vibrations to perform a plurality of substantially identical welding operations upon a workpiece.
Accordingly, there exists an industrial need for an improved, economical, space-saving, ultrasonic welding system having (1) a reduced number of components and (2 ) the capability of employing welding horns of varying configurations and masses which are adapted to weld workpieces of varying thicknesses and materials, and (3) having but a single r.f. power generator. The present invention provides a method and apparatus for economically and reliably performing a plurality of ultrasonic welding operations upon a workpiece, said workpiece including portions of varying thickness and/or portions fabricated from varying materials. This is accomplished by providing a welding system in which the multiconfigured and multimassed plurality of welding horns are energized by a single r.f. power generator, each horn being associated with a mechanism for transmitting an individually tuned acoustic signal thereto, whereby it is possible to deliver an optimum supply of ultrasonic vibrations to each of the workpieces. To that end, the present invention is further equipped with a sequencing mechanism which is adapted to sequentially provide an individually tuned (impedence matched) r.f. signal to each of a plurality of transducers. In short, described hereinbelow is an invention which fulfills a long-felt industrial need for an economical, space-saving, multihorn, ultrasonic welding system adapted to provide individualized doses of ultrasonic energy to the welding horns thereof so as to weld the workpiece.
These and further objects and advantages of the present invention will become clear from the drawings, the detailed description of the invention and the claims which are found herein.